1. Field of the Invention
The present invention relates to a flat coreless DC motor, and more particularly to a flat coreless vibrator motor for supporting a vibration calling mode in a mobile communication device.
2. Description of the Related Art
Generally, coreless DC motors are called xe2x80x9cmoving coil DC motorsxe2x80x9d. Such coreless DC motors are mainly classified into cylindrical and disc types. Disc type coreless DC motors are also classified into pancake motors, printed motors, and three-coil motors.
Among such DC motors, three-coil motors, which have a light structure, are generally used for a miniature, lightness, and low power consumption of mobile communication devices. Three-coil motors developed at an early stage are disclosed in Japanese Patent Laid-open Publication No. Sho. 63-290140 and Japanese Patent Application No. Hei. 4-295503. Disclosed in Japanese Patent Laid-open Publication No. Sho. 63-290140 is an eccentric plate mounted to the output shaft of a DC motor having a general construction. Japanese Patent Application No. Hei. 4-295503 discloses a rotor in which three armature coils eccentrically arranged to form a flat sector shape without overlapping with one another.
Now, a conventional coreless DC motor including a sector-shaped rotor will be described in conjunction with FIGS. 1 and 2. FIG. 1 is a sectional view illustrating the conventional flat coreless DC motor. FIG. 2 is a plan view of the conventional flat coreless DC motor as shown in FIG. 1. As shown in FIGS. 1 and 2, the flat coreless DC motor includes an eccentric rotor 9 having armature coils a, b, and c eccentrically arranged, and a shaft 7 arranged at a center of the eccentric rotor 9. The flat coreless DC motor also includes a housing having a case 1 for supporting the eccentric rotor 9 while allowing the eccentric rotor 9 to rotate freely, and a bracket 2. The flat coreless DC motor further includes a magnet 3 for supplying a magnetic flux to the eccentric rotor 9, a brush 6 arranged inside the magnet 3 under the condition in which a brush base 4 is interposed between the magnet 3 and brush 6, and a commutator 10 arranged on a surface of the eccentric rotor 9 facing the magnet 3. The brush 6 is slidable along the commutator 10 at its one end while being in contact with the commutator 10. In FIGS. 1 and 2, the reference numeral 8 denotes a separation preventing holder made of brass. Where a separation preventing means such as a joint is provided at the shaft 7, the separation preventing holder 8 may be dispensed with. The reference numeral 11 denotes a synthetic resin molded on the eccentric rotor 9. Typically, a glass fiber reinforced polybutylene terephtalate. The reference numerals 13a and 13b denote thrust washers, respectively.
However, since recent research made in association with such motors is to provide a miniature and light structure, those motors cannot generate a sufficient centrifugal force. Furthermore, in the case of the sector-shaped rotor, there is a limitation on an increase in centrifugal force because it is difficult to provide a space for a weight, such as lead beads, to be integrally formed at the rotor.
Another conventional type of rotor is also disclosed in Japanese Patent Laid-open Publication No. Sho. 63-290140 issued on Nov. 28, 1988. In this publication, a rotor structure is disclosed, in which a part of three armature coils uniformly spaced apart from one another by 120xc2x0 is eliminated. FIGS. 3a and 3b illustrate two rotor structures disclosed in the publication. FIG. 3c illustrates an armature coil circuit of the conventional motor. FIG. 3d is a graph depicting torque characteristics of the conventional motor. As shown in these drawings, the conventional motor is driven in a general fashion, that is, in a 3-phase 120xc2x0 constant current conduction fashion. In the case of a general 3-phase 120xc2x0 3-coil type coreless motor, a high efficiency, for example, an average torque of 95.5% and a torque ripple of 13.4%, is obtained because torque ripples generated at respective phases are equal. However, the above mentioned motor exhibits a degraded efficiency, for example, an average torque of 87.5% and a torque ripple of 56.7, because a non-uniformity in resistance occurs at one of the phases during the operation of the motor by virtue of the elimination of one coil, as shown in FIGS. 3c and 3d. As a result, the centrifugal force generated during the rotation of the motor is reduced, thereby resulting in vibrations of a degraded quality having a high frequency.
As apparent from the above description, it is difficult to provide a sufficient space for installing a weight adapted to obtain an increased centrifugal force, in the case of the conventional 3-coil type vibrator motor. Where a part of coils is eliminated, there are problems in that vibrations of a degraded quality having a high frequency are generated due to the formation of an idle point or a reduction in average torque.
Therefore, an object of the invention is to solve the above mentioned problems and to provide a vibrator motor capable of preventing the formation of an idle point, thereby obtaining a high average torque, and thus, a high efficiency, while providing a sufficient redundant space for installing a weight adapted to obtain an increase in centrifugal force.
In accordance with the present invention, this object is accomplished by providing a coreless DC motor comprising: a stator including a flat field magnet having a plurality of alternate N and S magnetic poles; a rotor spaced apart from the field magnet by a predetermined space to rotate freely, the rotor including at least three coreless armature coils and a commutator connected to the coils, at least two of the coreless armature coils being integrally formed together at a same position to form a non-coil section in the rotor; and a brush for supplying a current to the coils via the commutator of the rotor.
In accordance with the present invention, the armature coils having the double coil structure may have a size larger than that of the remaining coils.